help mechanics stumped

[comanche32]

well guys, the mechanic had my truck for 2 weeks now and can't figure it out. it's a 90 comanche with the 4.0. it starts up fine, will go up and down the hiway fine, but doesn't want to idle very long. he says its getting plenty of fuel and fire when it dies he thinks it's a sensor. he's tested the iac and the tps and says they're good. but he can't figure out what's wrong. any help would be greatly appreciated. i'm ready to have my jeep back.

[jimoshel]

Just a wild guess but check the exhaust, See if the Cats clogged.

[Blue88Comanche]

check the relays while you are at it.. i forgot i removed one to fix my other car while my MJ was waiting on a new tranny.. any way it would not idle right off i had to give it some gas for a short bit before it would idle.

 

the relay i replaced in the car was broke, it just wore out i guess

[mjeff87]

how did he test the IAC? I would try pulling it and cleaning it first.

[Boots]

:agree: clean the hole it goes in really good, they like to stick.

[1987Comanche]

Does it not want to idle when it's cold? The fuel mapping in the OE Renix ECU was flawed and Chrysler/Mopar issued an updated ECU that had the correct tables. I had to 2 foot my '90 4.0 out of the development every morning until I was lucky enough to stumble upon the updated ECU at the boneyard. I would never have guessed this without finding the TSB. Have him look up TSB 18-54-89 Revision A.

[comanche32]

thanks for the suggestions guys I forgot to mention that I cleaned the IAC before I took it in and the hole. I'm stumped to driving me and the mechanic crazy

[Dadinator]

Does it not want to idle when it's cold? The fuel mapping in the OE Renix ECU was flawed and Chrysler/Mopar issued an updated ECU that had the correct tables. I had to 2 foot my '90 4.0 out of the development every morning until I was lucky enough to stumble upon the updated ECU at the boneyard. I would never have guessed this without finding the TSB. Have him look up TSB 18-54-89 Revision A.

 

How do you identify it?

[carnuck]

Sounds to me like either the TPS is out of adjustment or there is a vacuum leak at the intake (EGR sticking open?)

[comanche32]

someone told me that it could be the crankshaft sensor does that sound right

[Eagle]

someone told me that it could be the crankshaft sensor does that sound right

No.

 

It might be the temperature sensor. Not the one in the head that tells your dashboard gauge how hot the engine is -- I'm talking about the one on the left side of the block that tells the ECU how hot the engine is.

 

When the Renix starts up, until the engine reaches operating temperature (around 165 to 170 degrees) it ignores sensors such as the oxygen sensor and runs on a pre-mapped, extra rich fuel curve. Once the temp comes up, it switches out of "open loop" mode into "closed loop" mode, in which it responds to sensor input. My guess is that, if your temp sensor is bad (or, God forbid, your ECU), you are stuck in warm-up mode and running too rich. That extra rich fuel curve could be too much for the engine to handle at warm idle.

[1987Comanche]

How do you identify it?

 

The TSB has the ECU #s listed but, as a quick and dirty method, look for the MOPAR tag indicating it's a reman ecu.

[offroader461]

If i were you i would check the actual fuel pressure with a gauge. My truck did the exact same thing and i finally checked the fuel pressure. At idle it was fine but as soon as i gave it some throttle it sputtered and almost quit running. I pulled the fuel pump and it turned out to be the rubber hose that runs from the pickup assembly to the acutal pump itself. The hose had rotted out and under load the fuel was just dumping right back into the tank and starving the truck. Check that before you start wasting money on parts. :cheers:

[dfreeman616]

my first thought was egr stuck or possibly some carbon partially keeping the valve open, but vacuum leak, coolant temp sensor that goes to the computer, or ecu itself are all other good possibilities.

[cruiser54]

Try these instructions for checking the TPS. Be sure to do your testing on the flat 3-wire connector.

 

 

RENIX TPS ADJUSTMENT

 

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.

It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

 

IMPORTANT NOTE: Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector

of the TPS . The letters are embossed on the connector itself.

Touch the black lead of your meter to the negative battery post.

If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be

necessary. The harness repair must be performed before proceeding.

I can provide an instruction sheet for that if needed.

 

MANUAL TRANSMISSION:

 

RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.

This manual transmission vehicle TPS provides data input to the ECU. The manual transmission

TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.

 

Wire "A" is positive.

Wire "B" is ground.

 

Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..

Note the voltage reading--this is your REFERENCE voltage.

 

Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your

OUTPUT voltage.

Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For

example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If

you can't achieve the correct output voltage replace the TPS and start over.

 

AUTOMATIC TRANSMISSION:

 

RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-

wire connector, same as the manual transmission vehicles have, and it is tested the same as the

manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.

 

However, the automatic TPS also has a square four-wire connector clearly embossed with the letters

A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module.

 

Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your

REFERENCE voltage.

 

Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT

voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.

For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.

If you can't, replace the TPS and start over.

 

So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and

the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire

connector side of the TPS. If you have ENGINE issues check the three-wire connector side of

the TPS.

 

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is

stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.

 

 

[comanche32]

thanks guys looks like its up to me to figure it out mechanic gave up on it.

[carnuck]

It could be a corroded firewall connector too. AKA C101

[cruiser54]

It could be a corroded firewall connector too. AKA C101

 

C101 is only on 87 and 88s, but it sure can be a source of many problems.

 

For those interested, here is a rough draft of instructions on how to clean the C101.

 

Renix Jeep C101 Connector Refreshing

 

 

The C101 connector on 1987 and 1987 Renix Jeeps was a source of electrical resistance when the vehicles were new. So much so that the factory eliminated this connector in the 1989 and 1990 models. The factory recommended cleaning this connector to insure the proper voltage and ground signals between the ECU and the fuel injection sensors. We can only imagine how this connector has become a larger source of voltage loss and resistance increase over a period of almost 25 years.

 

Almost every critical signal between the engine sensors, injectors, and the ECU travel the path through the C101.

 

The C101 is located on the driver’s side firewall above and behind the brake booster. It is held together with a single bolt in it’s center. To get the connectors apart, simply remove the bolt and pull the halves apart. You will find the connector is packed with a black tar like substance which has hardened over time.

Take a pocket screwdriver or the like and scrape out all the tar crap you can. Follow up by spraying out both connector halves with brake cleaner and then swabbing out the remainder of the tar. Repeat this procedure until the tar is totally removed. This may require 3 or more repetitions. Wipe out the connectors after spraying with a soft cloth.

 

If you have a small pick or dental tool tweak the female connectors on the one side so they grab the pins on the opposite side a bit tighter. Apply a true dielectric grease to the connection and bolt it back together.